Introduction to Photoelectron Spectroscopy
Photoelectron spectroscopy is a method that uses the photon—a subatomic particle of light—to investigate the properties of matter. Spectroscopy, in general, can use any part of the electromagnetic spectrum to probe matter, but photoelectron spectroscopy specifically uses a photon to interact with an electron. The most commonly used photons in photoelectron spectroscopy have ultraviolet and X-ray wavelengths of light. Photoelectron spectroscopy is also known as photoemission spectroscopy, because the information it supplies depends on the energy emitted by the electron that has interacted with the photon.
Photoelectron spectroscopy relies on the photoelectric effect to reveal information about matter. Matter is made up of atoms and an atom is composed of a nucleus and one or more electrons. In the basic model of the atom, the electrons move around the nucleus, grouped by distance from it; these groupings occur in atomic orbitals. The number of atomic orbitals dictates the properties of the element made up of these atoms. Electrons closer to the nucleus are core electrons; electrons farther away from the nucleus and involved in chemical bonds are valence electrons. The photoelectric effect is a phenomenon that occurs when an electron is propelled out of its atomic orbital after absorbing energy from a photon. This happens when the energy of the photon is greater than the potential of that atomic orbital. When the electron emits this energy, scientists can measure it and infer information about the atomic orbital—and thus, the material itself.
There are multiple types of photoelectron spectroscopy—sometimes abbreviated as PES—each with different names and acronyms. These varieties are a function of the photon type used to probe a material. Ultraviolet photons are used to energize valence electrons (those in bonds), and photoelectron spectroscopy using ultraviolet photons is referred to as UPS. X-ray photons are used to energize core electrons, and photoelectron spectroscopy using X-ray photons is referred to as XPS. Older scientific papers may use the acronym ESCA—electron spectroscopy for chemical analysis—when discussing how photoelectron spectroscopy can identify elements. The following sections discuss the specific applications of these different photoelectron spectroscopy techniques.
Development of Photoelectron Spectroscopy
Photoelectron spectroscopy began to be widely applied in the 1960s. This investigative technique is relatively new because it depends on an understanding of light (the electromagnetic spectrum) not realized until the early twentieth century.
While the concept of light as a spectrum was discovered by Isaac Newton in the 17th century, the photoelectric effect—the key underpinning of photoelectron spectroscopy—was not completely understood until a 1905 paper by Albert Einstein explained that light traveled in quanta, or units. Earlier, in 1887, German physicist Heinrich Hertz had documented the photoelectric effect, noting that ultraviolet light could produce sparks when it hit a metal plate.
Einstein's work built on the efforts of a number of European physicists during the 19th century who designed apparatuses to investigate the relationship between light and electromagnetic waves, such as Hertz, Alexandre-Edmond Becquerel, and Aleksandr Stoletov. After the publication of the 1905 Einstein paper, it took a few decades for scientists to harness the photoelectric effect, because it required them to design and refine spectroscopes to generate photons of only a single wavelength (such as ultraviolet and X-ray photons) as well as detectors to measure the resulting emitted energies.
Equipped with photoelectron spectroscopes, scientists investigated matter using these two sources (ultraviolet photons and X-ray photons). Ultraviolet photoelectron spectroscopy was developed by multiple groups. Key teams included one in Russia, led by Feodor I. Vilesov in the 1960s, that investigated gas molecules, and another in the United Kingdom, led by David Turner, that developed the prototype of the apparatus still in use today to perform ultraviolet photoelectron spectroscopy. In the 1950s, Kai Siegbahn began working on what would become a prototype of the contemporary X-ray photoelectron spectrometer. In his 1967 book Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy, he and his coauthors laid out the field of electron spectroscopy for chemical analysis, which is known as X-ray photoelectron spectroscopy today. Siegbahn was awarded a Nobel Prize for this work.
Today, photoelectron spectroscopy is applied to fields as varied as energy storage and the recycling of important elements such as carbon and nitrogen by geological processes.
Importance of Photoelectron Spectroscopy
Photoelectron spectroscopy is an important tool because it allows scientists to investigate the properties of matter (gases, solids, and liquids) through measurement of the orbital energies of electrons. From this information, scientists can infer specifics about the elements of a material, as well as about the bonds between the atoms in the material. Photoemission spectroscopy is a very accurate technique for measuring these electron energies.
Photoelectron spectroscopy is best used to infer surface information about materials, but each type of photoelectron spectroscopy has its own strength. X-ray photoelectron spectroscopy focuses on core electrons. It can reveal the elements that make up a material, as well as whether there are bonds between those elements. X-ray photoelectron spectroscopy is used to investigate the properties of matter across many scientific fields. Historically, ultraviolet photoelectron spectroscopy was an important method for experimentally confirming the orbital energies predicted by quantum mechanics, because it measures energy from valence electrons. Currently, ultraviolet photoelectron spectroscopy is used in solid state and condensed matter physics to measure the electron density of states for organic and inorganic materials. Angle-resolved photoemission spectroscopy is used in condensed matter physics to measure the electron energies of crystalline solids. Finally, extreme-ultraviolet photoelectron spectroscopy allows scientists to measure the conductivity of materials.
A simple example of photoelectron spectroscopy’s use is in determining the element that comprises a material. First, a scientist irradiates the material with a photoelectron spectroscope and measures the resulting spectrum. This spectrum will include a number of peaks at specific energy values. The number of peaks corresponds to the number of atomic orbitals in the unknown element. The scientist can use the number of peaks to tell how many atomic orbitals are in the material. From basic chemistry, the scientist knows the number of orbitals and the order in which electrons fill those orbitals. Elements with five peaks would have electrons in their 1s, 2s, 2p, 3s, and 3p atomic orbitals. So, this material is made of an element that has electrons in the 3p orbital. Next the scientist examines the relative values of the peaks themselves to determine how many electrons are in each orbital. In this example, the 3p orbital has a smaller relative value than the 2s orbital, indicating that there is only one electron in the 3p orbital. By referencing the periodic table, the scientist can conclude that this material is aluminum, which has only one electron in the 3p orbital.
Photoelectron spectroscopy has been used for analysis in fields such as geology and biochemistry, as well as more traditional fields of materials science. The applications of X-ray photoelectron spectroscopy are wide, from medical (confirming a sufficient amount of antibacterial silver in a wound dressing) to energy storage (understanding the degradation of solar cells) to basic chemistry (phase transitions of matter and crystal nucleation). Ultraviolet photoelectron spectroscopy was originally applied, and continues to be used, to characterize the gas phase of many molecules.
Benefits of Photoelectron Spectroscopy
The main benefit of photoemission spectroscopy is the measurement of highly accurate values for electron energies. These energy values allow scientists to infer information about the molecule from which the electron was emitted.
A second benefit of photoelectron spectroscopy is that other physical processes occur along with the photoelectric effect. These processes include X-ray fluorescence and the Auger process. In both of these processes, a higher-energy electron moves to the atomic orbital vacated by the electron that was dislodged through the photoelectric effect. The energy emitted when this (initially) higher-energy electron changes atomic orbitals provides additional information about the material under investigation.
A third benefit is this tool's ability to reveal information about the bonds of a molecule. The energy required to knock a core electron from its atomic orbital has some dependency on the electrons involved in the bonds of that molecule. Thus, using X-ray photoelectron spectroscopy to measure the energy required to dislodge a core electron allows scientists to infer information about the bonds of the molecule.
Limitations of Photoelectron Spectroscopy
Photoelectron spectroscopy allows for learning things about the interiors of atoms and molecules by probing the electrons that are part of them. To dislodge an electron from its atomic orbital, allowing its emitted energy to be measured when it moves back into that orbital, the energy source must impact the electron with a sufficiently energized photon. Valence electrons—those involved in chemical bonds—can be dislodged with ultraviolet photons, but core electrons require the energies of X-ray photons. Thus, the main limitation of photoemission spectroscopy is generating the energy required to dislodge the type of electron that corresponds to the information being sought.
Additionally, photoelectron spectroscopy is limited to information about the surfaces of materials, because it is dependent on the dislodged photoelectron traveling from the material to the detector, which grows less likely the more material the electron must travel through. Other limitations of photoelectron spectroscopy touch on the design of the spectrometer itself. Photoelectron spectrometers must be able to create a single type of energy source (either ultraviolet or X-ray photons). The spectrometers also must function within a vacuum.
New Developments in Photoelectron Spectroscopy
There are multiple avenues of new photoelectron spectroscopy development at Pacific Northwest National Laboratory (PNNL), including ultrafast time-resolved spectroscopy, improvements in energy resolution, and a new low-temperature imaging system.
Time-resolved spectroscopy is the application of existing spectroscopy techniques to a chemical process—when a molecule or material is changing—rather than a material in equilibrium. Future developments at PNNL include the application of existing photoelectron spectroscopy to study the dynamics of excited states of clusters of ions, such as aerosols nucleating to form clouds in the atmosphere.
Another initiative at PNNL aims to improve the resolution of the photoelectron spectroscope. The resolution of the spectroscope is a function of its components, such as the electron analyzer and the monochromatic source. The purpose of the electron analyzer is to measure the energy of the dislodged electron. The analyzer's resolution decreases as the energy of the electron increases unless additional components, such as optical lenses, are added to the spectroscope.
Additionally, a PNNL team is working on designing a cryo-XPS system to probe highly volatile materials, such as liquid electrolytes in a battery. Using this novel setup, the team will investigate specific regions of the battery after different charging and discharging conditions (without dismantling the battery).
Photoelectron Spectroscopy in Use at Pacific Northwest National Laboratory
PNNL performs basic science research with the aim of innovating sustainable energy technology. This includes collaborating with academia and industry to share the benefits of their research results as widely as possible. Photoelectron spectroscopy is used in many fields as an investigative tool, with applications in climate studies, biophysics, and energy storage.
A team at PNNL is using this tool to understand how aerosols form in our atmosphere. Aerosols are important to climate change studies: They are involved in the formation of clouds, as well as the absorption and transmission of the Sun's energy through the atmosphere. While scientists understand how an existing aerosol molecule can grow into a cluster large enough to be the seed for a cloud, the initial formation of the cluster is not well understood. The team developed an integrated approach involving synthesizing clusters of atmospherically relevant ions and molecules—such as α-pinene, a pine tree molecule known to aid in the formation of aerosol clusters—in the laboratory with electrospray ionization. They characterized the size and composition of the clusters using mass spectrometry. They used photoelectron spectroscopy to investigate the structures, energetics, interaction strengths, thermodynamics, and kinetic properties of the aerosol prenucleation clusters. Using photoelectron spectroscopy allowed the team to quantify interactions between atmospherically relevant ions and molecules, providing critical molecular-level information to better understand the process involved in aerosol particle formation. This work was done in collaboration with the Chinese Academy of Sciences.
Another team at PNNL uses photoelectron spectroscopy to investigate key molecules involved in different aspects of biophysics. One research effort characterized the energy level relating to the green fluorescent protein, a marker that is attached to important molecules. The protein’s green glow highlights a molecule visually so scientists can monitor it during biochemical reactions. The team, in collaboration with colleagues at Louisiana State University, used photoelectron spectroscopy to determine the energy level of the excited state of the cells that produce the protein’s color. A second biophysical research effort used photoelectron spectroscopy to characterize the structure of boron-based compounds. These compounds play a vital role in neutron capture therapy for treating brain tumors. In order to pass through cell walls, the compound needs to be nested within a cyclodextrin molecule. This nested form, called an inclusion complex, results from bonds between the compound and the molecule. The team used photoelectron spectroscopy to characterize the stability of this complex as a function of amounts of boron-based compounds and cyclodextrin.
The energy storage research team has developed a novel in situ XPS chemical imaging system to analyze the reactivity of ions produced at electrochemical interfaces. Their current research focuses on evaluating and predicting degradation of electrochemical materials used in a wide variety of batteries, such as redox flow, lithium-metal, and multivalent ion batteries.